Mobile radio systems are useful for providing reliable communications for many different types of applications, such as, military, public safety (e.g., police, fire, emergency medical technicians, and the like), or within other organizations or businesses with dispatch requirements (e.g., taxis, utilities, trucking, and the like). Often, individual radios (or subscriber units) are organized or otherwise assigned to a group for purposes of group calling, where a message from one radio in the group is broadcast and received by other radios in the group.
In some situations, a user may desire to have his or her message provided to users of radios assigned to multiple different groups. For example, in an emergency, a police officer may desire to communicate with other public safety personnel (e.g., firefighters and/or emergency medical technicians) in addition to the other members of the police department. To support a group call among multiple different radio groups, existing systems typically require either a centralized controller or a special messaging protocol to establish a temporary group by reprogramming the individual radios. Additionally, over-the-air interference or being outside of the broadcast range during the setup procedure may result in some radios being left out of the temporary group, and as a result, one or more users may miss hearing the message.